Many natural products which contain a chromophoric group capable of absorbing ultraviolet light undergo structural changes when exposed to sunlight. Such transformations are known either to bring about the formation of biologically active compounds or to bring about the destruction of such materials, or both. Of particular interest in this present research are natural materials which contain the dienic and trienic chromophoric grouping since these groupings are related to the formation and the destruction of the antirachitic factor, vitamin D. The role played by the triene unit in the biological activity of the vitamin will be evaluated, particular attention being given to the polarity, to the pi-bonding ability, and to the hydrogen bonding ability of substituted vitamin D analogs. Such studies should help define the nature of the active site of the hormonal metabolites of the vitamin. Studies will be undertaken which should aid in the identification and the understanding of the chemistry involved in the formation and in the transformations of vitamin D. The role of "sudden polarization" in the light-induced reactions of a variety of unsaturated biologically-active natural materials will be examined.